The present invention relates generally to channel quality estimation in wireless networks and, more particularly, to a method of generating channel quality estimates for a later time interval based on feedback received from a mobile terminal in an earlier time interval.
High Speed Downlink Packet Access (HSDPA) is a packet data service offered in Wideband Code Division Multiple Access (WCDMA) networks. The HSDPA provides high data rates using enhanced features such as higher order modulation, physical layer hybrid automatic repeat requests, multicode transmission, fast link adaptation, fast scheduling, and spatial multiplexing.
The physical channel for HSDPA is a time and code multiplexed channel shared by a plurality of mobile terminals. The mobile terminals periodically send channel quality indication (CQI) reports to the base station. The CQI reports indicate the instantaneous channel conditions as seen by the receiver. Based on the CQI reports from the mobile terminals, a scheduler at the base station schedules the downlink transmissions to the mobile terminals and determines the transmission format for the downlink transmissions. The transmission format includes, for example, the transport block size, modulation, and coding, which are selected to achieve a desired error performance. The identity of the mobile terminals scheduled to receive data in a given time interval, and the transmission format, is transmitted to the mobile terminals over a shared downlink control channel.
In HSDPA networks, the channel quality reports are known as channel quality indication (CQI) reports. The CQI report includes a channel quality indication that describes the effective channel quality as perceived by the demodulator in the mobile terminal. The CQI report essentially conveys a standardized measure of the SIR of the received signal after demodulation. The CQI report is sent periodically by the mobile terminal, while the HSDPA connection is active, even when the mobile terminal is not currently receiving data on the shared downlink channel.
While the CQI reports are useful in scheduling transmissions to mobile terminals and selecting the proper transmission format, the interference conditions as seen by the receiver may change between the time that the mobile terminal sends the CQI report to the base station, and the time that the mobile terminal is scheduled to receive a transmission on the downlink. The amount of both own-cell and other-cell interference may change. For example, interference sources present in the time interval when the CQI report is made may no longer be present in the time interval when the mobile terminal is scheduled to receive downlink transmissions. Further, interference sources present in the time interval when the mobile terminal is scheduled to receive downlink transmissions may not have been present in the time interval when the CQI was reported to the base station. These variations in interference sources can result in a mismatch between the selected transmission format and the current channel conditions when the downlink transmissions are made.
In order to still be able to use past CQI reports that have been affected by variations in the instantaneous interference patterns, smoothing may be applied to average out the time varying interference patterns. The use of smoothing means that any fast channel variations are also smeared, which can significantly impede the scheduler's ability to follow instantaneous channel fading for optimal resource allocation.
A related problem is that the mobile terminal itself may apply a certain extent of smoothing to the SIR estimates used to generate the CQI report before mapping the estimated SIR to a corresponding CQI. Smoothing of the SIR estimates is used to reduce the SIR estimation variance. In order to avoid averaging out the channel variations, the smoothing should be limited to a few slots or time intervals, which in itself should be sufficient for handling the channel-related component of the SIR measurement. However, the presence of additional interference requires heavier smoothing in order to reduce the CQI report variance. Again, this reduces the usefulness of the reported CQI values for fast scheduling and efficient resource allocation.
Accordingly, a CQI reporting method is still needed that allows the scheduler to obtain a better estimate of the channel quality experienced by a mobile terminal in the time interval when the downlink transmissions to the mobile terminal are made.